This invention relates to hydraulically supported thrust bearings, and in particular it relates to hydraulically supported thrust bearings for large vertical-shaft dynamoelectric machines
Thrust bearings for large vertical-shaft machines usually comprise a downwardly facing ring that is mounted to the rotating component of the machine and a plurality of shoes or bearing segments with upwardly directed bearing surfaces. The bearing segments support the rotating ring on an oil film which is usually provided by having the segments and the ring submerged in an oil bath.
In order to align the surface of the shoes with the surface of the rotating ring, it is known to mount each shoe on a pivot located near the centre of the shoe and arranged to permit the shoe to tilt down slightly at the leading edge to form the oil film into a slight wedge-shaped configuration. This tends to improve the load carrying capability and to reduce the temperature of the components. In its basic form, however, this arrangement may be vulnerable to conditions which cause the total bearing load to be unevenly apportioned amongst the individual shoes. Some effort has been directed to alleviating this difficulty, as for example, by the provision of a system of equalizing levers. One example of the use of equalizing levers is described in U.S. Pat. No. 2,565,116 to Baudry, issued Aug. 21, 1951.
There is another problem involved in the use of a basic pivotted shoe design, and that arises because each shoe is supported near the geometrical centre of its lower surface. An ideal oil film pressure distribution on the upper or bearing surface of each shoe would extend substantially to the perimeter of the shoe, although decreasing in intensity as the perimeter is approached. The ideal pressure distribution is difficult to achieve. Because of the central position of the shoe support, and a more general oil film pressure distribution, the shoe tends to distort so that its upper surface has a generally convex upward shape. Since this is a departure from the desirable planar condition, the oil film pressure profile and the operating characteristics of the bearing are degraded. In addition to this, there tends to be a thermal gradient within the shoe. That is, the shoe tends to be hotter at the top than at the bottom. This thermal gradient will tend to reinforce the distortion towards forming a convex surface. The larger the radial or circumferential dimensions of the shoe, the greater is the tendency for the shoe surface to crown or become slightly convex.
It is also known to support each shoe on a plurality of springs distributed beneath each shoe. The springs are designed with sufficient resilience to enable each shoe to tilt slightly as if it were effectively on a pivot and so that the total bearing load is distributed on and between each shoe. Because the shoe support is not concentrated near the centre of the shoe, but provides a distributed support pattern, the shoe can be made thinner without causing an undesirable crowning of the shoe. Because the shoe can be made thinner and more flexible, the tendency to crown or distort due to thermal gradients is reduced and the ability of the shoe to adapt to shape changes in the rotating ring is improved. However, the formation of an optimum oil pressure profile may be hindered because the distribution of shoe support now tends to be too strong towards the periphery of the shoe thus causing the bearing surface of the shoe to take a concave shape.
Another known thrust bearing design uses a single hydraulic element beneath each shoe. Canadian Pat. No. 695,080--Block et al, issued Sept. 29, 1964 describes one arrangement of hydraulic thrust bearing support by way of example. The hydraulic element can be in the form of a cylinder with a piston or can be a chamber formed with flexible walls such as bellows. The hydraulic elements are connected together by a manifold to ensure that the total thrust bearing load will be evenly distributed between all the shoes. Further, the hydraulic element may be designed with a certain limited amount of angular flexibility so that it can perform the function of a pivot. This small amount of pivotting action permits the shoe surface to align itself with the plane of the rotating ring as well as permitting sufficient tilt to form a hydrodynamic oil film. However, as with mechanical pivot support referred to above, there are problems in avoiding the tendency to crown, that is in avoiding the tendency of the shoe surface to become convex with the resulting degradation of the oil film pressure profile.
In order to compensate for uneven deflection of the inclinable support member, a thrust bearing arrangement is described in Canadian Pat. No. 1,116,671--Starcevic, issued Jan. 19, 1982, which includes a plurality of mechanical support elements for each bearing shoe. A mechanical pivot supports a relatively thick support member or backing plate for each shoe. The shoe, which is thinner than the support member, is supported above the pivoted support member by a plurality of mechanical support elements which extend between the support member and the bottom of the shoe. Each support element is designed with a predetermined compressive stiffness so that deflections in the support member under load are compensated for thus purportedly providing a bearing surface which is substantially planar.